1. Field of the Invention
The present invention relates to a device for controling vaporized fuel in an internal combustion engine provided with a charcoal canister.
2. Description of the Prior Art
An internal combustion engine is usually provided with a device for cutting the fuel when the engine is in a deceleration condition so as to prevent overheating of a catalytic converter or the generation of so-called after fire. An internal combustion engine is also usually provided with a charcoal canister for temporarily holding the fuel vaporized from the fuel tank or float chamber of the carburetor. The canister is connected to the carburetor at a position located downstream of the throttle valve so that the fuel absorbed in the charcoal layer in the canister is desorbed due to the flow of purge air generated by the intake vacuum downstream of the throttle valve.
In such an internal combustion engine which provides both a fuel cutting system during deceleration and a system for the absorption and desorption of fuel by the canister, after fire would take place during deceleration if the fuel from the canister were introduced into the engine. In the prior art, the passageway from the canister is opened to the carburetor at an intake port (purge port) located slightly upstream of the throttle valve in its idle position. In this construction, the purge port is located upstream of the throttle valve during deceleration so that the introduction of fuel from the canister is prevented. However, the prevention of the introduction of fuel from the canister during deceleration renders ineffective operation of the canister during a running condition wherein the engine frequently experiences a number of alternate acceleration and deceleration operations, which condition is often realized when a vehicle is being operated in a city. This causes a certain amount of vaporized fuel to be emitted into the atmosphere, i.e., the vaporized fuel is not caught by the canister.
In order to overcome this drawback, there is known a system wherein the purge port is always located downstream of the throttle valve so that the introduction of vaporized fuel is attained even if the engine is under deceleration, the purge port including a control valve (purge control valve) for controlling the amount of vaporized fuel to be introduced (see Japanese Unexamined Patent Publication No. 53-74620). This system has, however, a drawback in that the catalytic converter can become overheated and after fire is apt to be generated at deceleration with a high engine speed due to the introduction of vaporized fuel.